WP4C and Water Potential
Water potential is a measurement of the energy status of the water in a system. It indicates how tightly water is bound, structurally or chemically, within a substance. Water potential can be computed from the vapor pressure of air in equilibrium with a sample in a sealed measurement chamber. For a more detailed description of water potential, please refer to Chapter 9, titled “Theory: Water Potential” of this manual.
How the WP4C works
WP4C uses the chilled-mirror dewpoint technique to measure the water potential of a sample. In this type of instrument, the sample is equilibrated with the headspace of a sealed chamber that contains a mirror and a means of detecting condensation on the mirror. At equilibrium, the water potential of the air in the chamber is the same as the water potential of the sample. In the WP4C, the mirror temperature is precisely controlled by a thermoelectric (Peltier) cooler. Detection of the exact point at which condensation first appears on the mirror is observed with a photoelectric cell. A beam of light is directed onto the mirror and reflected into a photodetector. The photodetector senses the change in reflectance when condensation occurs on the mirror. A thermocouple attached to the mirror then records the temperature at which condensation occurs. Values begin to be displayed indicating that initial measurements are being taken. WP4C then signals you by flashing a green LED and/or beeping when final values are reached. The final water potential and temperature of the sample is displayed.
In addition to the technique described above, WP4C uses an internal fan that circulates the air within the sample chamber to reduce time to equilibrium. Since both dewpoint and sample surface temperatures are simultaneously measured, the need for complete thermal equilibrium is eliminated.
The WP4C controls the sample temperature by means of an internal thermo-electrical module that monitors and stabilizes the sample block temperature according to how it is set.
WP4C and Water Potential
Water potential is a measurement of the energy status of the water in a system. It indicates how tightly water is bound, structurally or chemically, within a substance. Water potential can be computed from the vapor pressure of air in equilibrium with a sample in a sealed measurement chamber. For a more detailed description of water potential, please refer to Chapter 9, titled “Theory: Water Potential” of this manual.
How the WP4C works
WP4C uses the chilled-mirror dewpoint technique to measure the water potential of a sample. In this type of instrument, the sample is equilibrated with the headspace of a sealed chamber that contains a mirror and a means of detecting condensation on the mirror. At equilibrium, the water potential of the air in the chamber is the same as the water potential of the sample. In the WP4C, the mirror temperature is precisely controlled by a thermoelectric (Peltier) cooler. Detection of the exact point at which condensation first appears on the mirror is observed with a photoelectric cell. A beam of light is directed onto the mirror and reflected into a photodetector. The photodetector senses the change in reflectance when condensation occurs on the mirror. A thermocouple attached to the mirror then records the temperature at which condensation occurs. Values begin to be displayed indicating that initial measurements are being taken. WP4C then signals you by flashing a green LED and/or beeping when final values are reached. The final water potential and temperature of the sample is displayed.
In addition to the technique described above, WP4C uses an internal fan that circulates the air within the sample chamber to reduce time to equilibrium. Since both dewpoint and sample surface temperatures are simultaneously measured, the need for complete thermal equilibrium is eliminated.
The WP4C controls the sample temperature by means of an internal thermo-electrical module that monitors and stabilizes the sample block temperature according to how it is set.
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